Charles Henley

FGF23 neutralization improves chronic kidney disease–associated hyperparathyroidism yet increases mortality

Chronic kidney disease-mineral and bone disorder (CKD-MBD) is associated with secondary hyperparathyroidism (HPT) and serum elevations in the phosphaturic hormone FGF23, which may be maladaptive and lead to increased morbidity and mortality. To determine the role of FGF23 in the pathogenesis of CKD-MBD and development of secondary HPT, we developed a monoclonal FGF23 antibody to evaluate the impact of chronic FGF23 neutralization on CKD-MBD, secondary HPT, and associated comorbidities in a rat model of CKD-MBD. CKD-MBD rats fed a high-phosphate diet were treated with low or high doses of FGF23-Ab or an isotype control antibody. Neutralization of FGF23 led to sustained reductions in secondary HPT, including decreased parathyroid hormone, increased vitamin D, increased serum calcium, and normalization of bone markers such as cancellous bone volume, trabecular number, osteoblast surface, osteoid surface, and bone-formation rate. In addition, we observed dose-dependent increases in serum phosphate and aortic calcification associated with increased risk of mortality in CKD-MBD rats treated with FGF23-Ab. Thus, mineral disturbances caused by neutralization of FGF23 limited the efficacy of FGF23-Ab and likely contributed to the increased mortality observed in this CKD-MBD rat model.

Repeated administration of vanilloid receptor TRPV1 antagonists attenuates hyperthermia elicited by TRPV1 blockade

Capsaicin, the active ingredient in some pain-relieving creams, is an agonist of a nonselective cation channel known as the transient receptor potential vanilloid type 1 (TRPV1). The pain-relieving mechanism of capsaicin includes desensitization of the channel, suggesting that TRPV1 antagonism may be a viable pain therapy approach. In agreement with the above notion, several TRPV1 antagonists have been reported to act as antihyperalgesics. Here, we report the in vitro and in vivo characterization of a novel and selective TRPV1 antagonist, N-(4-[6-(4-trifluoromethyl-phenyl)-pyrimidin-4-yloxy]-benzothiazol-2-yl)-acetamide I (AMG 517), and compare its pharmacology with that of a closely related analog, tert-butyl-2-(6-([2-(acetylamino)-1,3-benzothiazol-4-yl]oxy)pyrimidin-4-yl)-5-(trifluoromethyl)phenylcarbamate (AMG8163). Both AMG 517 and AMG8163 potently and completely antagonized capsaicin, proton, and heat activation of TRPV1 in vitro and blocked capsaicin-induced flinch in rats in vivo. To support initial clinical investigations, AMG 517 was evaluated in a comprehensive panel of toxicology studies that included in vivo assessments in rodents, dogs, and monkeys. The toxicology studies indicated that AMG 517 was generally well tolerated; however, transient increases in body temperature (hyperthermia) were observed in all species after AMG 517 dosing. To further investigate this effect, we tested and showed that the antipyretic, acetaminophen, suppressed the hyperthermia caused by TRPV1 blockade. We also showed that repeated administration of TRPV1 antagonists attenuated the hyperthermia response, whereas the efficacy in capsaicin-induced flinch model was maintained. In conclusion, these studies suggest that the transient hyperthermia elicited by TRPV1 blockade may be manageable in the development of TRPV1 antagonists as therapeutic agents. However, the impact of TRPV1 antagonist-induced hyperthermia on their clinical utility is still unknown.

The calcimimetic AMG 641 abrogates parathyroid hyperplasia, bone and vascular calcification abnormalities in uremic rats

Calcimimetics and vitamin D sterols reduce serum parathyroid hormone (PTH) in patients with secondary hyperparathyroidism receiving dialysis, a disease state associated with parathyroid hyperplasia, vascular calcification, bone disease, and increased mortality. The aim of this study was to determine the effects of the research calcimimetic AMG 641 (Amgen, Inc., Thousand Oaks, CA) or calcitriol (Sigma Aldrich Corporation, St. Louis, MO) on vascular calcification in a rodent model of progressive uremia with accompanying secondary hyperparathyroidism induced by dietary adenine. Treatment effects on parathyroid gland hyperplasia and bone loss were also investigated. Rats were treated daily with vehicle, calcitriol (10 ng), AMG 641 (3 mg/kg), or no treatment during the 4 week period the animals were fed adenine. The uremia-induced increases in serum PTH levels were significantly attenuated by both AMG 641 (>90%) and calcitriol (approximately 50%). AMG 641 significantly reduced calcium-phosphorus product (CaxP) and significantly attenuated the development of both parathyroid hyperplasia and vascular calcification. In addition, AMG 641 prevented the defects in trabecular bone volume, trabecular number, and bone mineralization, as well as increases in trabecular spacing in this rodent model of secondary hyperparathyroidism. Calcitriol (10 ng/rat) decreased osteoid surface/bone surface, but had no effects on other bone parameters, or parathyroid hyperplasia (likely due to the lower PTH suppressive effect of calcitriol at the dose used in this study). However, this dose of calcitriol significantly exacerbated vascular calcification. These results suggest that calcimimetics can reduce the development of vascular calcification, parathyroid hyperplasia and bone abnormalities associated with secondary hyperparathyroidism.

Effect of Calcium and the Calcimimetic AMG 641 on Matrix-Gla Protein in Vascular Smooth Muscle Cells

Vascular calcification (VC) is frequently observed in patients with chronic renal failure and appears to be an active process involving transdifferentiation of vascular smooth muscle cells (VSMCs) to osteoblast-like cells. Reports of VC prevention in uremic rodents by calcimimetics coupled with identification of the calcium-sensing receptor (CaSR) in VSMCs led us to hypothesize that CaSR activation in arterial cells and VSMCs may elicit expression of an endogenous inhibitor of VC. Toward this end, we determined the effects of calcium and the calcimimetic AMG 641 on arterial wall and isolated VSMC expression of matrix-Gla protein (MGP). Bovine VSMCs were incubated with increasing calcium chloride or AMG 641 concentrations, while in vivo experiments were carried out on healthy and uremic rats. Both AMG 641 and hypercalcemia induced MGP expression in the arterial wall in healthy and uremic rats. The results obtained in vitro supported those from in vivo experiments. In conclusion, selective CaSR activation, either by extracellular calcium or AMG 641, increased MGP expression in vivo in the arterial wall and in vitro in bovine VSMCs. This local upregulation of MGP expression provides one potential mechanism by which calcimimetics prevent VC.

Calcimimetic Inhibits Late-Stage Cyst Growth in ADPKD

In polycystic kidney disease (PKD), genetic mutations in polycystin 1 and 2 lead to defective intracellular trafficking of calcium, thereby decreasing intracellular calcium and altering cAMP signaling to favor proliferation. We hypothesized that calcimimetics, allosteric modulators of the calcium-sensing receptor, would reduce cyst growth by increasing intracellular calcium. We randomly assigned 20-wk-old male rats with a form of autosomal dominant PKD (heterozygote Cy/+) to one of four groups for 14 to 18 wk of treatment: (group 1) no treatment; (group 2) calcimimetic R-568 formulated in the diet; (group 3) R-568 plus calcium-supplemented drinking water (R-568 plus Ca); or (group 4) Ca-supplemented drinking water with a normal diet (Ca). Severity of PKD did not progress in any of the three treatment groups between 34 and 38 wk. Compared with no treatment, cyst growth was unaffected at 34 wk by all treatments, but cyst volume and fibrosis were lower at 38 wk, with both R-568-treated groups demonstrating a greater reduction than calcium alone. Between 34 and 38 wk, the total kidney weight increased by 78% in the control group (P < 0.001) and by 19% in the Ca group (P < 0.01), but did not increase in the R-568 or R-568 plus Ca groups, suggesting inhibition of disease progression despite equivalent suppression of parathyroid hormone. In summary, treatment of hyperparathyroidism halts late-stage progression of rodent cystic kidney disease. The benefit of R-568 alone suggests calcium-sensing receptor modulation may have additional inhibitory effects on late-stage cyst growth resulting from a direct modulation of intracellular calcium.

Calcification inhibitors and Wnt signaling proteins are implicated in bovine artery smooth muscle cell calcification in the presence of phosphate and vitamin D sterols.

Administration of active vitamin D sterols to treat secondary hyperparathyroidism in patients with chronic kidney disease receiving dialysis has been associated with elevated serum calcium and phosphorus levels, which may lead to increased risk of vascular calcification. However, calcimimetics, by binding to the parathyroid gland calcium-sensing receptors, reduce serum parathyroid hormone, calcium, phosphorus, and the calcium-phosphorus product. Using cultured bovine aorta vascular smooth muscle cells (BASMCs), an in vitro model of vascular calcification, we compared calcification levels and gene expression profiles after exposure to the phosphate source ß-glycerolphosphate (BGP), the active vitamin D sterols calcitriol and paricalcitol, the calcimimetic R-568, or BGP with the active vitamin D sterols or R-568. Cells exposed to BGP (10 mM) alone or with calcitriol or paricalcitol showed dose-dependent BASMC calcification. No change in calcification was observed in cultures exposed to BGP with R-568, consistent with the observed lack of calcium-sensing receptor expression. Microarray analysis using total cellular RNA from cultures exposed to vehicle or BGP in the absence and presence of 10−8 M calcitriol or paricalcitol for 7 days showed that cells exposed to BGP with calcitriol or BGP with paricalcitol had virtually identical gene expression profiles, which differed from those of cells treated with BGP or vehicle alone. Several osteoblast- and chondrocyte-associated genes were modulated by BGP and vitamin D exposure. In this study, exposure of BASMCs to phosphate and active vitamin D sterols induced calcification and changes in expression of genes associated with mineralized tissue.

R-568 reduces ectopic calcification in a rat model of chronic kidney disease-mineral bone disorder (CKD-MBD)

BACKGROUND Chronic kidney disease-mineral bone disorder (CKD-MBD), a newly defined disorder in patients with CKD, describes the interacting triad of (1) biochemical abnormalities of calcium, phosphorus and parathyroid hormone (PTH), (2) extraskeletal calcification and (3) abnormal bone. METHODS We studied the effects of the calcimimetic R-568, R-568 with calcium (R-568 + Ca) or calcium (Ca) alone compared with control CKD rats on this triad in the Cy/+ male rat, a model of progressive CKD that spontaneously develops CKD-MBD on a normal phosphorus diet. Animals were treated for either 14 or 18 weeks beginning at 20 weeks of age (34-week and 38-week animals, respectively). RESULTS The results demonstrated similar efficacy of R-568, R-568 + Ca and Ca in lowering PTH levels. R-568 alone lowered plasma calcium compared to control over time, but increased phosphorus compared to control early in the course of the disease, but not at 38 weeks. Animals treated with Ca alone or R-568 + Ca had lower phosphorus levels; the Ca alone group had elevated Ca levels. Bone volume improved in the calcium-treated groups. In contrast, arterial and cardiac calcification worsened by most assessments in the R-568 + Ca and Ca alone treated animals compared with R-568 alone whereas R-568 alone treatment showed beneficial effects on most sites of extraskeletal calcification. CONCLUSION Thus, R-568, with or without Ca, improved the biochemical abnormalities of hyperparathyroidism but with higher and lower calcium levels, respectively, compared with controls. However, R-568 + Ca had more dramatic improvement in bone volume, but more extraskeletal calcification than R-568 alone. This complexity demonstrates that treatment of one component of CKD-MBD may lead to undesirable effects on other components.

Cinacalcet HCl prevents development of parathyroid gland hyperplasia and reverses established parathyroid gland hyperplasia in a rodent model of CKD

Background. Secondary hyperparathyroidism (sHPT) represents an adaptive response to progressively impaired control of calcium, phosphorus and vitamin D in chronic kidney disease (CKD). It is characterized by parathyroid hyperplasia and excessive synthesis and secretion of parathyroid hormone (PTH). Parathyroid hyperplasia in uremic rats can be prevented by calcium-sensing receptor (CaSR) activation with the calcimimetic cinacalcet (Sensipar®/Mimpara®); however, it is unknown, how long the effects of cinacalcet persist after withdrawal of treatment or if cinacalcet is efficacious in uremic rats with established sHPT. Methods. We sought to determine the effect of cinacalcet discontinuation in uremic rats and whether cinacalcet was capable of influencing parathyroid hyperplasia in animals with established sHPT. Results. Discontinuation of cinacalcet resulted in reversal of the beneficial effects on serum PTH and parathyroid hyperplasia. In rats with established sHPT, cinacalcet decreased serum PTH and mediated regression of parathyroid hyperplasia. The cinacalcet-mediated decrease in parathyroid gland size was accompanied by increased expression of the cyclin-dependent kinase inhibitor p21. Prevention of cellular proliferation with cinacalcet occurred despite increased serum phosphorus and decreased serum calcium. Conclusions. The animal data provided suggest established parathyroid hyperplasia can be reversed by modulating CaSR activity with cinacalcet and that continued treatment may be necessary to maintain reductions in PTH.

The calcimimetic AMG 641 accelerates regression of extraosseous calcification in uremic rats.

The purpose of the present study was to test the hypothesis that extraskeletal calcification regresses in uremic rats after reduction in phosphorus intake and treatment with calcimimetics. Extraosseous calcification was induced in five to six nephrectomized rats fed a high-phosphorus (1.2%) diet who received calcitriol (80 ng/kg ip) every other day for a period of 14 days. Next, dietary phosphorus was reduced to 0.6%, and rats were treated with vehicle (n = 20), calcitriol [80 ng/kg ip/48 h (n = 20)], or the calcimimetic AMG 641 [1.5 mg/kg sc/48 h (n = 20)]. Aortic and soft-tissue calcium and phosphorus content was evaluated after 14 and 28 days. At 28 days, reduction of phosphorus intake resulted in a significant decrease in tissue mineral content in vehicle- and AMG 641-treated rats but not in rats receiving calcitriol. Aortic calcium and phosphorus was lower in rats treated with AMG 641 (96.7 +/- 26.4 mg/g) than in rats receiving vehicle (178.3 +/- 38.6 mg/g). An infiltrate of phagocytic cells expressing the calcium-sensing receptor was identified in areas surrounding foci of calcification. Additional studies in parathyroidectomized rats demonstrated that AMG 641 increased the urinary excretion of calcium (6.2 +/- 0.6 vs. 3.1 +/- 0.5 mg/day, vehicle) (P < 0.001). In conclusion, experimentally induced extraosseous calcification in uremic rats can be partially resolved by reducing phosphorus intake; the addition of calcimimetics may accelerate the regression process through mechanisms potentially involving a direct stimulatory effect on mineral phagocytic cells plus an increase in urinary calcium excretion.

Discovery of a Calcimimetic with Differential Effects on Parathyroid Hormone and Calcitonin Secretion

Calcimimetics are positive allosteric modulators to the calcium-sensing receptor (CaSR). Activation of the CaSR inhibits the secretion of parathyroid hormone (PTH), stimulates the secretion of calcitonin, and decreases serum calcium (Ca2+). Cinacalcet, a second-generation calcimimetic, is used therapeutically to control PTH in patients with chronic kidney disease who are on dialysis with secondary hyperparathyroidism. A calcimimetic that displays increased separation of PTH versus Ca2+ lowering in patients would potentially allow the use of calcimimetics to treat patients in earlier stages of renal disease because hypocalcemia can develop in this population. Toward this end, we developed a third-generation calcimimetic, determined the molecular pharmacological properties of it using an operation model of allosteric modulation/agonism, and measured the compound effects on PTH, serum ionized Ca2+, and calcitonin levels in 5/6 nephrectomized rats. We found the new molecule effectively reduced PTH levels without promoting calcitonin secretion or hypocalcemia. Furthermore, our third-generation molecule was less efficacious at promoting calcitonin secretion from human thyroid carcinoma cells compared with 3-(2-chlorophenyl)-N-((1R)-1-(3-methoxyphenyl)ethyl)-1-propanamine (R-568), a first-generation calcimimetic. These data provide evidence that calcimimetics with increased potency can be used to lower PTH without production of significant hypocalcemia because the threshold for inhibition of PTH secretion is much lower than the threshold for calcitonin secretion.